Ion guides are known wherein ions are confined or constrained to move along the central longitudinal axis of a linear ion guide. The central axis of the ion guide is coincident with the centre of a radially symmetric pseudo-potential valley. The pseudo-potential valley is formed within the ion guide as a result of applying RF voltages to the electrodes comprising the ion guide. Ions enter and exit the ion guide along the central longitudinal axis of the ion guide.
Ion guides are also known having first and second linear ion guide portions which are conjoined. Ions are confined or constrained to move along the central axis of the first ion guide portion of the ion guide or along the central axis of a second ion guide portion of the ion guide. Ions can be radially transferred back and forth from the first ion guide portion to the second ion guide portion as desired by applying a potential difference between the two ion guide portions.
A problem with the known ion guide having first and second linear ion guide portions which are conjoined is that the DC potential profile varies abruptly across the ion guide in a radial direction from the first ion guide portion towards the second ion guide portion. As a result, ions which are close to the boundary region between the first and second ion guide portions may experience a very strong electric field which could undesirably cause the ions to become activated and/or fragmented. Conversely, ions which are further away from the boundary region may experience only a very weak electric field and hence may not be transferred from the first ion guide portion to the second ion guide portion.
It is therefore desired to provide an improved ion guide and method of guiding ions.